132 HORTICULTURE FOR SCHOOLS 



phosphate; or it is treated with sulfuric acid to form a com- 

 pound known as super-phosphate. Raw rock phosphate be- 

 comes available to plants very slowly, and there is little 

 appreciable effect from it until after a period of years. 

 Super-phosphate, on the other hand, dissolves readily, and 

 becomes available to the plant at once. However, it is very 

 much more expensive than raw rock phosphate. Ground 

 bone is a by-product of the meat industry. Before being 

 ground the bones are usually steamed in order to remove the 

 substances used in the manufacture of gelatine and glue. 

 This steaming process also dissolves out the fat, thus making 

 the final product very much more desirable for fertilizing 

 purposes; for if the fat is not removed, it forms a thin film 

 about each particle of bone and so hinders the process of 

 decay in the soil as to render the bone-meal practically use- 

 less for fertilizing purposes for a considerable period of time. 

 The steaming process is of further benefit in that the appli- 

 cation of heat hastens the breaking up of the phosphate com- 

 pounds of the bone, thus making some of the phosphorus 

 immediately available to the plant. 



Phosphorus is also obtained from Thomas slag, a by- 

 product in the refining of iron ore, containing from 15 to 20 

 per cent of phosphoric acid. It is ground to a fine powder 

 before being used as a fertilizing material. 



220. Potash. A considerable portion of the potash used 

 in the United States is imported from foreign countries, 

 although mineral deposits have been found in this country. 

 Potash is also secured as a by-product in the manufacture of 

 cement. The dust from the mills is collected and treated in 

 such a way as to extract the potassium compounds from it. 

 This not only yields more than sufficient potassium sulfate to 

 pay for the process, but also to a great extent eliminates the 

 very considerable injury formerly done by the cement dust 

 in killing plants growing in the vicinity. 



Soils derived from a granite source are likely to be rich in 



